Carbohydrates represent the largest fraction of biomass and various strategies for their efficient use as a feedstock for commercial chemicals are being established. Biomass is of particular interest due to its potential for supplementing, and ultimately replacing, petroleum. One such commercial chemical obtainable from biomass is lactic acid; a lactic acid derivative, methyl lactate, is a convenient building block towards renewable and biodegradable solvents and polymers.
Lactic acid derivatives, in particular esters of lactic acid, may be obtained from sugars via a variety of reaction processes including biochemical (enzymatic fermentation) and synthetic (catalytic conversion). Particular attention has been focussed on synthetic routes as they provide a commercially and environmentally advantageous alternative to biochemical routes.
EP 2 184 270 B1 and Holm et. al. Science (2010), 328, p 602-605 disclose the conversion of common sugars to lactic acid derivatives wherein a heterogenous zeotype or zeolite catalyst is employed. Specifically, the conversion of glucose, sucrose or fructose to methyl lactate is disclosed. A variety of heterogenous Lewis-acid zeotype catalysts are identified as particularly active catalysts for this conversion. Sn-BEA has been identified as one of the most selective catalysts, as illustrated by the conversion of glucose, fructose and sucrose to methyl lactate with yields of 43, 44 and 68% respectively. There is a desire to further improve the percentage yield of useful products such as lactate esters and esters of 2-hydroxy-3-butenoic acid for this process.
In addition to the desire to improve the product yield of such processes using Sn-BEA as the catalyst, an increase in the product yield using alternative, easier to synthesize metallo-silicate catalysts would broaden the scope of catalysts suitable for industrially scaled processes. A particular advantage would be the application of an improved process and improved catalytic activity wherein the preparation of the catalyst itself is simplified and made suitable for industrial scale production.
Zeotype and zeolite catalysts comprising an active metal can be prepared by several methods including direct synthesis and post-synthesis preparations.
Direct synthesis methods, as described in EP 1 010 667 B1 and Chem Commun. (1997) pp 425-426, although very convenient for laboratory scale synthesis, may not be suitable for industrial scale production. These direct synthesis procedures experience practical and environmental limitations posed by: the use of reagents comprising fluoride such as hydrogen fluoride (HF); lengthy synthesis times; and the use of expensive organic templating agents which are difficult to reuse.
Recently, Hermans et al Angew. Chem. Int. Ed. (2012), 51, p 11736-11739, disclosed the preparation of Sn-BEA using a post synthesis process that does not require the use of a fluoride reagent. The post synthesis process disclosed by Hermans comprises de-aluminating a commercially available zeolite (Al-BEA), physically mixing this de-aluminated zeolite with a tin salt, followed by calcination of the de-aluminated zeolite and tin salt mixture. A general post-synthesis preparation method should thus include a step of defect generation in the silica framework via de-alumination or other similar procedure, for example de-boronation; and the addition of tin (Sn) and calcination, which produces the incorporation of Sn in the silica structure.
The post-synthesis preparation process has the advantages of: a short synthesis time; the possibility that large amounts of active metal may be incorporated into the framework structures; and avoidance of the use of expensive organic templating and a reagent comprising fluoride. These advantages enable the process of preparing the Sn-BEA to be scalable and potentially suitable for industrial scale processes. A similar post-synthesis methodology has also been applied to the synthesis of Sn-MWW: Qiang et al. ChemSus-Chem [vol 6, issue 8, 1352-1356, August (2013)].
For industrial scale catalyst production, it is a significant advantage if the metallo-silicate catalysts can be prepared by a post synthesis process; however, so far reported, all catalysts prepared by the post synthesis process are poor catalysts for the conversion of C6 sugars to lactate esters and esters of 2-hydroxy-3-butenoic acid. Only low yields of the desirable esters are obtained. See Example 1 of the present application.
Therefore there is a desire to improve both the percentage yield of the process of preparing lactic acid and 2-hydroxy-3-butenoic acid or esters thereof from sugars, in particular wherein the preparation of the catalyst is industrially feasible. It would be an additional advantage if the improvement could also broaden the scope of active catalysts suitable for this process and for an industrial scale.